This invention relates in general to the field of integrated circuit field effect transistors and the effects of threshold associated therewith as reflected to an electrical signal source coupled thereto and more particularly, to a dual field effect transistor structure which reduces the effects of these threshold voltages on the electrical signal sources coupled thereto.
Charge coupled devices (CCD's) are presently being used, in some cases, to provide memory storage arrays for digital or analog information derived from a corresponding array of measuring detectors. In the field of infrared (IR) imaging, for example, wherein photodetectors are used as the measuring detectors, it is generally desirable to have injection type structures fabricated at the inputs of the CCD arrays to couple the information measured from the IR photodetectors to the CCD arrays which provide storage of the time dependent photo images formed. However, many of these conventional injection type structures are comprised of a metal oxide semiconductor (MOS) field effect transistor (FET) which exhibit variations in their threshold voltages between inputs. The threshold voltages of the injection type CCD inputs depend on several parameters which are nonuniform across a given Si wafer of CCD's and are generally affected by integrated circuit (IC) fabrication processes. Larger variation of the threshold voltages are expected between CCD injection type inputs on different wafers as compared to CCD inputs on the same wafer. It appears that variation of parameters upon which the threshold voltage (V.sub.T) is dependent increases over larger areas. Examples of the magnitude of threshold variations between different conventional detector/CCD inputs as measured experimentally on several CCD coupling structures are illustrated in Table 1. The offset voltage variations, for the best CCD in Table 1 is .+-.42 mV about the mean threshold voltage. The offset voltage between injection diffusion inputs fabricated on different wafers is larger (.about.250 mV) than the offset variations of inputs fabricated on the same wafer.
In the cases when the measuring detectors are current sources, such as back-biased photodiodes, for example, the voltage potential applied across the current source must be at least at a minimum value to render the current source operable across its measuring spectrum. For economy, the photodiodes are normally back-biased by a common voltage source. Under these conditions, the threshold voltage variations of conventional MOSFET injection type input structures make it difficult to adjust the common back-biasing voltage source at a mean value to assure proper operability of all the current source measuring detectors. In addition, these threshold variations may also affect certain operating performance characteristics of the measuring detectors, such as impedance, noise and signal level, for example. Therefore, it appears that before direct injection becomes a viable coupling scheme, the effects of the threshold voltage variations on the detector's operation should cause no degradation to its performance.
TABLE I __________________________________________________________________________ Injection Input No. CCD NUMBER (different wafers) (Same Wafer) 2 3 5 10 14 15 20 21 __________________________________________________________________________ 20 -42 30 -54 6 21 -132 -12 42 19 42 -21 -120 -48 -30 -87 -15 24 18 -75 0 -80 -39 -60 -117 12 18 17 -69 -21 -78 -54 -78 -144 -42 24 16 -72 -30 -63 -81 -78 -126 -51 -21 15 -84 -39 -93 -84 -81 -117 -51 -39 14 -54 -39 -63 * -78 -96 -48 -21 13 -42 -6 -54 * -24 -93 -51 -21 12 -45 -24 -84 -78 -63 -120 -51 -30 11 -75 -9 -42 -36 -57 -96 -60 18 10 -33 -24 -24 -33 -6 -99 -60 15 9 -15 48 -9 -24 0 -87 -72 51 8 -3 39 -42 -15 -18 -93 -63 48 7 * 27 -33 -6 -57 -60 -42 30 6 -39 9 -15 6 -36 -90 -33 9 5 -12 -72 -6 -18 -42 -54 -30 24 4 -3 0 12 -12 -27 6 -21 42 3 18 27 -57 -3 -21 39 -21 54 2 -3 60 -9 0 -12 51 -18 -6 1 0 0 0 0 0 0 0 0 __________________________________________________________________________ *Very large offsets
Theoretically, it can be shown that a reduction in the threshold voltage of a MOSFET injection input structure can be effected by feedback techniques. However, because the relative offset voltage between any pair of injection inputs is unknown, it appears that a feedback arrangement may be required for each injection input of a CCD array. A desirable feedback arrangement may be one in which the threshold voltage of the injection input is compensated in such a manner as to substantially remove its effects on the operation of a detector coupled thereto. And further, if such an input structure could be characterized to have low noise, low power and be capable of operating at cryogenic temperatures, it could be suitable for the majority of analog signal processing applications employing CCD's. Such a feedback compensation is described herein in connection with the present invention.